Recently, devices for compressing and encoding an image by adopting an encoding scheme of handling image information digitally and performing compression by an orthogonal transform such as a discrete cosine transform and motion compensation using image information-specific redundancy for the purpose of information transmission and accumulation with high efficiency when the image information is handled digitally have become widespread. Moving Picture Experts Group (MPEG) and the like are examples of such encoding schemes.
Particularly, MPEG 2 (International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) 13818-2) is a standard that is defined as a general-purpose image encoding scheme, and covers interlaced scan images, progressive scan images, standard resolution images, and high definition images. MPEG 2 is now being widely used in a wide range of applications such as professional use and consumer use. Using the MPEG 2 compression scheme, for example, in the case of an interlaced scan image of a standard resolution having 720×480 pixels, a coding amount (bit rate) of 4 to 8 Mbps is allocated. Further, using the MPEG 2 compression scheme, for example, in the case of an interlaced scan image of a high resolution having 1920×1088 pixels, a coding amount (bit rate) of 18 to 22 Mbps is allocated. Thus, it is possible to implement a high compression rate and an excellent image quality.
MPEG 2 is mainly intended for high definition coding suitable for broadcasting but does not support an encoding scheme having a coding amount (bit rate) lower than that of MPEG 1, that is, an encoding scheme of a high compression rate. For example, with the spread of mobile terminals, it is considered that the need for such an encoding scheme will increase in the future, and thus an MPEG 4 encoding scheme has been standardized. An international standard for an image encoding scheme was approved as ISO/IEC 14496-2 in December, 1998.
Further, in recent years, standards such as H.26L (International Telecommunication Union Telecommunication Standardization Sector Q6/16 Video Coding Expert Group (ITU-T Q6/16 VCEG)) for the purpose of image encoding for video conferences have been standardized. H.26L requires a larger computation amount for encoding and decoding than in existing encoding schemes such as MPEG 2 or MPEG 4, but is known to implement high encoding efficiency. Further, currently, as one activity of MPEG 4, standardization of incorporating even a function that is not supported in H.26L and implementing high encoding efficiency based on H.26L has been performed as a Joint Model of Enhanced-Compression Video Coding.
As a standardization schedule, an international standard called H.264 and MPEG-4 Part 10 (Advanced Video Coding (hereinafter referred to as “AVC”) was established in March, 2003.
Furthermore, as an extension of H.264/AVC, Fidelity Range Extension (FRExt) including an encoding tool necessary for professional use such as RGB or 4:2:2 or 4:4:4 or 8×8 DCT and a quantization matrix which are specified in MPEG-2 was standardized in February, 2005. As a result, H.264/AVC has become an encoding scheme capable of also expressing film noise included in movies well and is being used in a wide range of applications such as Blu-Ray Discs (trademark).
However, in recent years, there is an increasing need for high compression rate encoding capable of compressing an image of about 4000×2000 pixels (also referred to as a 4K image), which is 4 times that of a high-definition image, or delivering a high-definition image in a limited transmission capacity environment such as the Internet. To this end, improvements in encoding efficiency have been under continuous review by Video Coding Experts Group (VCEG) under ITU-T.
In this regard, currently, in order to further improve the encoding efficiency to be higher than in AVC, Joint Collaboration Team-Video Coding (JCTVC), which is a joint standardization organization of ITU-T and ISO/IEC, has been standardizing an encoding scheme called High Efficiency Video Coding (HEVC). A committee draft that is a draft specification for the HEVC standard was issued in January, 2013 (see Non-Patent Literature 1).
In the case of HEVC, generally, information transmitted from an encoding side to a decoding side includes not only a syntax for an I slice but also a syntax element for a P slice or a B slice, that is, a syntax element related to an inter-screen process.
Meanwhile, in AVC or HEVC, there is a method in which one picture is divided into a plurality of tiles, and encoding and decoding are performed in units of tiles. In this case, processes such as lossless encoding/lossless decoding, prediction, an orthogonal transform/inverse orthogonal transform, and quantization/inverse quantization are performed in units of tiles (without depending on another tile).
Thus, for example, in the decoding process, encoded data undergoes lossless decoding in units of tiles, and the prediction and transform processes are performed in units of tiles. Here, a processing period of time of the lossless decoding depends on a data amount but the prediction and transform processes are substantially constant without being significantly affected by a data amount. For this reason, in order to absorb a variation in the processing period of time of the lossless decoding, it was necessary to accumulate a decoding result in a memory.